Chapter 26 – The Vertebrates Animal Evolution: Every animal is a combination of traits, some which are conserved from remote ancestors and others that are unique to its branch of the family tree. All chordates share four features at some point of their development: Notochord supports body Nervous system develops from dorsal nerve cord Embryos have pharynx with slits Embryos have tail that extends past anus Urochordata: Salps and tunicates Cephalochordata: Lancelets Craniates: Fishes, amphibians, reptiles, birds, mammals. Cartilaginous or bony chamber encloses and protects brain Trends in the Evolution of Vertebrates Shift from notochord to vertebral column Nerve cord expanded into brain Evolution of jaws Paired fins evolved, gave rise to limbs Gills evolved, gave rise to lungs Adaptations to life on land - reptiles Tough, scaly skin Internal fertilization 2 kidneys Amniote eggs Adaptive Radiation: The early reptiles were very successful, giving rise to modern reptiles, birds and mammals. Amphibians Animals in the class Amphibia were the first that were able to spend at least a portion of their life on land. The first amphibians were probably primarily aquatic, making occasional trips on land to escape predation or in search of food (especially the insects that were undergoing their own adaptive radiation explosion). Its adaptations to living on land include a denser skeleton and appendages modified for moving on land and supporting its weight, improved lungs, more complex circulatory system (3 chambered heart), and improved hearing & vision and water conservation adaptations (e.g., more efficient kidneys). What is the geographic distribution of amphibians? What habitats do they inhabit? Temperate, sub-tropical and tropical latitudes of the world. Amphibians are often associated with streams or wetlands, including seasonal wetlands. Access to moist conditions or fresh standing or running water is important for amphibians. How do amphibians differ from fish? Name at least three adaptations that allow amphibians to live on land. Amphibians have lungs to breathe (pulmonary respiration) as well as cutaneous respirations. Fish generally lack lungs, and instead use gills for respiration. Many amphibians undergo complex life cycles while fish do not. Note that not all species of amphibians have a distinct larval stage (like a tadpole). These are known as “direct developers”. Amphibians have ears, eyes (including eyelids) that allow then to survive on land. They also can vocalize to communicate on land. Even though amphibians are terrestrial, they tend to be associated with water and/or moist habitats. Why? Water is essential to successful reproduction (e.g., some species of tropical tree frogs lay their eggs in rainwater captured in bromeliads in the forest canopy). The eggs are particularly vulnerable to desiccation. Larval forms of many amphibians live in aquatic habitats, even if the adult form is partially or completely terrestrial. In addition, amphibians can easily dehydrate due to their porous skin. Describe amphibian reproduction, including where fertilization takes place (internal or external), the type of eggs laid, and where egg-laying takes place. Amphibians reproduce sexually. Fertilization – the process of sperm uniting with egg occurs externally in most species of frogs (though the tailed frog in the Pacific Northwest is an important exception because the male deposits the sperm inside of the female). Females lay the eggs and the males deposit sperm in the same location. Some salamanders have internal fertilization in that males release packets of sperm, called spermataphores. Females then pick up the packet and inserts it into their cloaca where fertilization occurs. Most amphibians demonstrate no parental care. However, as usual, there are exceptions and some do guard the eggs and even feed young (e.g., the strawberry poison dart frog). Compare adaptations to terrestrial life and reproduction with that of the bryophytes (e.g. mosses) and seedless vascular plants (e.g. ferns). Bryophytes, like many amphibians are terrestrial, but are closely associated with water. Most amphibians lay their eggs in or near water and have external fertilization that is facilitated by water. Bryophytes, also require water to facilitate the movement of the flagellated sperm to fertilize the egg (usually rain or dew). Reptiles The class Reptilia includes turtles, snakes lizards, alligators and Komodo dragons. All of them have lungs to breathe on land and dry, scaly skin that does not need to be kept wet. What is the geographic distribution of reptiles? What habitats do they inhabit? Reptiles occur primarily in temperate and tropical latitudes, a couple of species occur near the Arctic region. Reptiles inhabit nearly all terrestrial habitats as well as marine and fresh water habitats. Describe reptile reproduction, including where fertilization takes place (internal or external), the type of eggs laid, and where egg-laying takes place. Reptiles primarily reproduce sexually. Fertilization is internal, thus making water unnecessary for the sperm to meet the egg. Most reptiles lay eggs, however a few snakes are live bearers. What is an amniote egg? How is production of an amniote egg an advantage over amphibian eggs? An amniote egg is one that has a protective covering that keeps it from desiccating. In addition, it contains three membranes used for protection, gas exchange and nourishment for the developing embryo. Reptiles, unlike amphibians, do not require water for their eggs allowing them to utilize many terrestrial habitats inaccessible to amphibians. What kind of parental care do reptiles demonstrate? Most reptiles demonstrate little or no parental care after laying their eggs and young are completely independent at hatching. However, some reptiles and snakes guard their nests and crocodiles guard their nests and hatchlings (though they don’t feed them). Incidentally, crocodiles are the most closely related reptile to birds. Compare reptilian adaptations to terrestrial life and method of reproduction with that of the gymnosperms. It is most useful to compare two characteristics of reptiles and gymnosperms. First, fertilization is internal in reptiles, negating the need for water to facilitate the movement of sperm (compare to amphibians). Pollen (containing the gymnosperm sperm) also does not need water and gymnosperms are wind pollinated. Next, the development of the amniote egg allowed early reptiles to lay their eggs on land. This parallels the development of the seed in plants, which protected the developing sporophyte embryo in a protective seed coat. Birds The class Aves includes all of the 9,800 recognized bird species in the world (of which only 930 are found in North America and 350 regularly occur in Oregon). Although northern states generally support fewer species of birds than southerly ones, Oregon ranks fifth behind only Florida, New Mexico, Texas, and California in terms of numbers of species. What is the geographic distribution of birds? What habitats do they inhabit? Birds have a worldwide distribution and occur in both polar regions at least part of the year. Consequently, they inhabit most terrestrial and marine habitats (including open ocean). Their ability to fly allowed them to colonize remote oceanic islands inaccessible to other vertebrate groups. Although some birds spend the majority of their lives at sea (e.g., albatross and penguins), all birds must come to land to lay their eggs. Name major adaptations birds evolved for flight. Important modifications to the skeleton, including the forelimb modifications to support wings. Overall there are a number of structural modifications (including pneumatization and fusion of many bones) that allow the skeleton to be simultaneously lightweight and strong. Large breast (pectoral) muscles and a keeled breastbone for attachment. Scales modified into lightweight and interlocking flight feathers that provide lift. Highly developed respiratory system to meet birds’ energetic needs during flight. Some scientists believe that birds are simply reptiles with feathers. Name 2 characteristics of birds that support this hypothesis (e.g. how are they similar to reptiles). Both have scales. Scales are obvious a bird’s feet, and feathers are derived from the same tissues as scales. Both lay eggs. The structure of eggs are very similar. Nucleated red blood cells (FYI – we, like all mammals, do not nuclei in our red blood cells). So-called transitional species from the fossil record, like Archaeopterix, show characteristics found in birds and reptiles. Also, there are a number of skeletal similarities that birds and reptiles have, but mammals do not. Describe avian reproduction, including where fertilization takes place (internal or external), the type of eggs laid, and where egg-laying takes place. Birds reproduce sexually. Fertilization is internal. Birds, like reptiles lay amniote eggs that have hard shells made of calcium carbonate. Some species do not build nests, while others build elaborate nests. Importantly, the eggs must be placed where they will be protected from the elements and predators and can be incubated. Some birds lay their eggs in the nests of other species (nest parasites). What kind of parental care do birds demonstrate? While a few species engage in little or no parental care (e.g., megapodes and nest parasites), the vast majority of birds both incubate eggs and care for nestlings through at least fledgling (and usually some time beyond fledging). Mammals Humans are members of the vertebrate class Mammalia. There are approximately 5000 mammal species in the world making up only about 0.4% of known animal species. What is the geographic distribution of mammals? What habitats do they inhabit? Birds have a worldwide distribution and occur in both polar regions at least part of the year. Consequently, they inhabit most terrestrial and marine habitats (including open ocean). Name traits that are unique to the class Mammalia. They produce milk for their young via mammary glands. They have hair. They have unique skeletal features (e.g., three bones in the middle ear, a single bone comprising the lower jaw). They have a diaphragm that separates the body cavity into two sections. There are both placental and non-placental mammals. Briefly describe how reproduction takes place in the following groups of mammals and give example of species for each. Mammals reproduce sexually (Dolly the sheep, aside). Fertilization is internal. 1. Placental mammals: Females give birth to live young. The developing fetus remains in the female and are provided nourishment via the placenta. We are placental mammals. 2. Monotremes: Mammals that lay eggs. Females actively care for eggs and have prolonged parental care. Duck-billed platypus, and echidnas. 3. Marsupials: marsupials have placentas and give birth to live young. However, the young are embryonic at birth and do most of their development attached to the female’s teat in a outer protective pouch. The only marsupial species in North America is the opossum. What kind of parental care do mammals demonstrate? Female mammals invest a lot of care into young by first carrying the developing young (marsupials and placental mammals) and then feeding them via mammary glands (all mammals), as well a protecting and educating young following birth. In humans, males are important providers of parental care. Biologists sometimes compare the angiosperms to the placental mammals. On what do they base this comparison? In placental mammals, the developing embryo is housed within the female’s uterus and nourished by the placenta. In angiosperms, The developing embryo is housed within the ovary and in some plants, the endosperm is critical to the development of the embryo. All of these comparisons provide a way to think about the evolution of plants and animals and how particular innovations have contributed to the success of these groups. Which of the four vertebrate classes rely solely on lungs to breathe (pulmonary respiration)? Which habitats are these classes likely to be found relative to the one that also uses cutaneous respiration? Terrestrial amphibians rely heavily on cutaneous respiration as well as lungs. Reptiles, birds and mammals breathe (that is, the exchange of O2 and CO2) via the lungs. The relatively waterproof skin of these vertebrate classes allows them to live in environments that would be far too desiccating for amphibians including very arid areas and marine habitats. Which of the four vertebrate classes are ectotherms, which are endotherms? In what way does this characteristic affect the general behavior and metabolism of each class? First, distinguish between ectotherm and endotherm. In ectotherms (cold-blooded), core body temperature is largely a function of the surrounding environment. The animal’s behavior often plays a critical role because its temperature depends so much on the ambient conditions (this is why basking is such an important reptile behavior). In endotherms (warm-blooded), the core body temperature is controlled largely by metabolic activity and physiological regulation of heat exchange with the environment. Behavior also is important (e.g., taking shelter from extreme temperatures), but temperature control is primarily physiological. Ectotherms tend to do better than mammals in nutrient- and oxygen-poor areas because their metabolic needs are less demanding that endotherms. This also explains why they can go such long periods without eating. Endotherms have high demands for oxygen, food and water to meet their energetic needs. So, for example, a foraging mouse uses 20 - 30 times more energy than a foraging lizard of equal mass. Consequently, endotherms are much more active than ectotherrms and much more likely to be nocturnal and active in winter. Which of the four vertebrate classes were found (or likely would be found) in the Polar Regions? Which ones were not (or unlikely would be)? Why? (think about adaptations). This is a follow-up question to the one above. Endotherms can be fully functional in extremely hot and cold environments, virtually everywhere on the planet, as long as there is sufficient oxygen, food and water. Mammals and birds have a variety of characteristics (fur, feathers, accumulation of body fat, large size) and behaviors (shivering, social behavior, hibernation, etc.) that help them maintain their body temperatures in extreme environments. In general, reptiles and amphibians live in temperate and tropical latitudes (both terrestrial and marine). Compare and contrast parental care among the four classes. In general, which of the classes have young that are more likely to survive to independence? Review the life-history patterns in section 45.5. The investment in parental care by mammals and birds relative to amphibians and most reptiles means that their young are more likely to survive to adulthood. So, they have fewer young over their lifetimes, but each one has a higher probability of surviving.